Textile designers are very creative. This is necessary because of seasonal factors and because the public taste continually changes, so the industry continually demands new products. Many designers in this industry would like the ability to custom-make their own yarns, so their products would be more unique, and so as to provide more flexibility in designing textiles.
Polyester (continuous) filament yarns have for many years had several desirable properties; but, hitherto, there has been an important limiting factor in the usefulness of most polyester flat yarns to textile designers, because only a limited range of yarns has been available from fiber producers, and the ability of any designer to custom-make his own particular polyester flat yarns has been severely limited in practice. The fiber producer has generally supplied only a rather limited range of polyester yarns because it would be more costly to make a more varied range, e.g. of deniers per filament (dpf), shrinkage properties, tensiles, and dyeability, and to stock an inventory of such different yarns.
Conventional flat polyester filament yarns used to be typically prepared, for example, by melt-spinning at low or moderate speeds to make undrawn yarns and then drawing and heating to increase tensiles (especially, modulus and yield point) and to decrease shrinkage. Conventional polyester filaments have combinations of properties that, for certain end-uses, could desirably be improved, as will be indicated hereinafter. It is important to recognize that what is important for any particular end-use is the combination of all the properties of the specific yarn (or filament), sometimes in the yarn itself during processing, but also in the eventual fabric or garment of which it is a component. It is easy, for instance, to reduce shrinkage by a processing treatment, but this modification is generally accompanied by other changes, so it is the combination or balance of properties of any filament (or staple fiber) that is important.
Generally, we refer herein to untextured filament yarns as "flat" yarns and to undrawn flat yarns as "feed" or as "draw-feed" yarns. Filament yarns which can be used as a "textile" yarn without need for further drawing and/or heat treatment are referred herein as "direct-use" yarns. For textile purposes, a "textile" yarn must have certain properties, such as sufficiently high modulus and yield point, and sufficiently low shrinkage, which distinguish these yarns from conventional feed yarns that require further processing before they have the minimum properties for processing into textiles and subsequent use. It will be recognized that, where appropriate, the technology may apply also to polyester filaments in other forms, such as tows, which may then be converted into staple fiber, and used as such in accordance with the balance of properties that is desirable and may be achieved as taught hereinafter.
From the parent application (now U.S. Pat. No. 5,066,447, the disclosure of which is hereby incorporated herein by reference), it is known that conventional polyester undrawn spin-oriented yarns (SOY) (and SOF, i.e., spin-oriented filaments) draw by a necking operation; i.e., that the undrawn polyester filaments have a natural draw-ratio NDR (and that drawing such polyester filaments at draw-ratios less than the NDR (herein referred to as partial-drawing) produces irregular "thick-thin" filaments which are considered inferior for most practical commercial purposes (unless a specialty yarn is required, to give a novelty or special effect). For filament yarns, the need for uniformity is particularly important, more so than for staple fiber. Fabrics from flat yarns show even minor differences in uniformity from partial drawing of conventional polyester undrawn SOY as defects, especially when dyeing these fabrics. Thus, uniformity in flat filament yarns is extremely important. Undrawn polyester filaments have been unique in this respect because nylon filaments and polypropylene filaments have not had this defect. Thus, it has been possible to take several samples of a nylon undrawn yarn, all of which have the same denier per filament, and draw them, using different draw-ratios, to obtain correspondingly different deniers in the drawn yarns, as desired, without some being irregular thick-thin filament yarns, like partially drawn polyester filament yarns.
So far as is known, it had not previously been suggested, prior to the parent application, that a draw process be applied to a polyester textile yarn, i.e., one that was itself already a direct-use yarn, such as having shrinkage and tensile properties that made it suitable for direct use in textile processes such as weaving and knitting without first drawing and heat setting. Indeed, to many skilled practitioners, it might have seemed a contradiction in terms to subject such a yarn to draw-warping, for example, because such a yarn was already a textile yarn, not a feed yarn that needed a drawing operation to impart properties useful in textile processes such as weaving or knitting.
According to the parent application, processes were provided for improving the properties of feed yarns of undrawn polyester filaments (especially undrawn polyester filament feed yarns that have the shrinkage behavior of spin-oriented polyester filaments such as have been disclosed by Knox in U.S. Pat. No. 4,156,071, and by Frankfort & Knox in U.S. Pat. Nos. 4,134,882 and 4,195,051 (discussed hereinafter). Such processes (according to the parent application) involve drawing with or without heat and with or without post heat-treatment, and are most conveniently adapted for operation using multi-end drawing, such as draw-warping; but such benefits may be extended to other drawing operations, such as preparing drawn flat yarns by split and coupled drawing of single-ends (or of a small number of ends, typically corresponding to the number of spin packages per winder or spin position of a small unit of winders) and to various draw (and no-draw) texturing processes for providing bulky filament yarns, such as by draw false-twist and air-jet texturing and no draw air-jet and stuffer-box texturing.